This invention relates to an imaging device for generating a two-dimensional image such as a fingerprint image.
Fingerprinting devices have been known for a number of years. In a simple version, a fingerprinting device may have a prism or another light-transmitting member having a first surface at which light transmitted through the member may undergo frustrated total internal reflection, a lens, a source of light emitting an interrogating beam, and an image sensor. A finger is placed on one of the surfaces of the prism, and the interrogating beam of light is directed onto another, illumination surface of the prism or the member so as to illuminate from within the prism or member the first surface on which the finger is placed. In accordance with a well-known principle of frustrated total internal reflection, the beam of light will be reflected from the finger-contacted surface substantially only at points where the finger does not touch the prism. The reflected light exits the prism or optical member through another surface thereof. Since, as is well known, the surface of any human finger is covered with characteristic ridges and recesses which create a unique pattern known as fingerprint, the reflected light which exits the prism carries the fingerprint image. The image can be sensed, retrieved and registered if necessary, using known means. The lens serves to focus the reflected beam of light to a certain degree to facilitate a further transmission of the beam.
In some devices, e.g. as described in U.S. Pat. No. 5,187,748, the illumination surface is at an acute angle to the first surface. In other cases, as seen for example in U.S. Pat. No. 5,233,404, the illumination surface is parallel to the first surface.
In a typical arrangement, illustrated herein in FIG. 1 as a Prior Art, the prism is triangular in cross-section. The finger under investigation is pressed against a first surface of the prism. An interrogating light beam is directed onto the first surface of the prism through another surface (illumination surface) of the prism and exits through a viewing principle of frustrated total internal reflection. The reflected beam passes through a lens (or a lens system) which is disposed at a distance from the prism, and is then directed onto an image sensor, e.g. a charge coupled device (CCD) or the like for converting the fingerprint image into an electric signal. It is evident that the image arriving at the lens is not a true image of the fingerprint as the surface of the finger in contact with the prism is not parallel to the lens (more specifically, to the plane of symmetry of the lens indicated with a dotted line on FIG. 1). Even when the light transmitting element is a plate, the analyzed object cannot be positioned parallel to the lens. Consequently, one end of the contact surface will be closer to the lens than the other end, and as a result, the fingerprint image will be somewhat distorted. The use of angled surfaces induces a perspective effect to measured biometric data. This perspective effect is very much the same as when a picture is angled away from a viewer. It can be said that the device of the above-defined type is inherently non-symmetrical.
It is known to use a diaphragm associated with the lens to prevent light propagating from the prism through the lens from reaching a peripheral portion of the lens where peripheral aberration typically occurs in standard lenses. The diagram has an opening in the centre that is coaxial with the optical axis of the lens and functions as an iris.
It is desirable to eliminate or correct for peripheral aberration and perspective distortion in optical fingerprint readers without a substantial reduction in imaged information. It is also desirable to reduce or eliminate the depth-of-field distortion that is due to the relative short optical path of the beam carrying the desired information. All the above corrections should desirably be effected, however, without a significant increase in the size of the apparatus.
In accordance with the invention, there is provided an optical imaging device comprising:
a light transmitting element having at least a first surface on which an imaged object can be placed and at which incident light beam transmitted through the light transmitting element may undergo frustrated total internal reflection, and a second surface through which light reflected from the first surface may exit as a reflected beam,
an image sensor disposed at a distance from the light transmitting element for receiving an image of said object from said light transmitting element,
a lens disposed in the light path of said reflected beam between the light transmitting element and the sensor for focusing said reflected beam and thus produce a focused beam, the lens having two dissimilar surfaces, a first refractive convex surface facing the incident reflected beam from the light-transmitting element, and a second surface having a significantly lesser curvature than said first convex surface, and
a reflective means disposed adjacent to said second surface for reflecting said reflected beam passed through said refractive surface, onto said image sensor so that an image of said object can be detected.
In keeping with the meaning of the word xe2x80x9cadjacentxe2x80x9d, the reflective surface (adjacent to the second surface) may be applied to the second surface or may be spaced therefrom by a distance not preventing the reflection of the incident beam back through the first surface and onto the sensor.
The second surface of the lens may be flat, or it may have a curvature that is significantly smaller than the curvature of the first, refractive surface that is turned towards the reflected beam from the light transmitting element. The curvature is not limited to spherical or regular aspheric. It should be selected to at least partly correct for the depth-of-field distortion of the system.
The light transmitting element may be embodied by a prism, a plate with parallel major surfaces, or a platen having one flat surface against which an analyzed object may be pressed and a plurality of parallel elongated prisms on the other surface.
Preferably, the reflective surface is sized to correspond only to a portion of the second surface, the portion defining a working area of the second surface about the optical axis of the lens. This serves to minimize the typical peripheral Gaussian distortion of spherical lenses that becomes larger as the light passes farther away from the optical axis of the lens i.e. closer to the periphery of the lens.
The device may further comprise other reflective surfaces in the optical path of said reflected beam between the light transmitting element and said sensor, to fold the optical path for space saving purposes.